Conservation Update: Oxygen Changes in the Inner Harbor

Baltimore’s Inner Harbor has always served as a main point of connection for locals and visitors alike—the water offers delightful views, commerce, recreation, and most importantly, a home for our local aquatic wildlife.

Published September 26, 2016

Earlier this month, the harbor experienced an environmental change that gave the water a vibrant pistachio hue and triggered blue crabs and fish to surface in search of oxygen. Thanks to constant monitoring of water quality, our experts were able to better understand what caused this change.

Two major water-related events occurred over the Labor Day weekend, causing the harbor to drastically change color and become depleted of dissolved oxygen. The pistachio color was a result of a thermal inversion or "turnover" event—a natural condition where surface water is cooled quickly causing it to sink. Warmer water from the bottom of the harbor then rises, bringing with it sulfur-loving bacteria naturally found in oxygen-deprived bottom sediments. These events typically occur in the spring and fall, when we experience hot days followed by cold nights. The now surfaced sulfur bacteria causes an odor noticed only second to the unnatural color.

To further complicate the matter, the harbor experienced a "mahogany tide" days prior to the thermal inversion. Named for its distinctive reddish-brown color, this chlorophyll-rich dinoflagellate peaked on August 29 and soon crashed with a major algae die-off (on the same day as the inversion).

Although an unfortunate event, these are important, visible reminders that our harbor is full of life. Over the last several years, the National Aquarium has documented animal diversity and abundance in Baltimore's Inner Harbor. Since first launching our Biohuts in 2014, we have confirmed 17 species of fish and crustaceans utilize these waters as their home. By understanding how water chemistry reacts during a thermal inversion or harmful algal bloom, we can better determine innovative ways to lessen the harmful effects to wildlife.